The art of lithographic printing is based on the immiscibility of ink and water. A lithographic printing plate is composed of ink receptive regions, commonly referred to as the “image area,” generated on a hydrophilic region on a substrate. When the surface of the printing plate is moistened with water and printing ink is applied, hydrophilic regions retain the water and repel the printing ink, and the image area accepts the printing ink and repels the water. The printing ink retained on the image area may then be transferred to the surface of a material upon which the image is to be reproduced. Typically, the ink is first transferred to an intermediate blanket, which in turn transfers the ink to the desired surface.
Lithographic printing plates typically comprise a radiation-sensitive coating applied over the hydrophilic surface of a substrate. Conventional radiation-sensitive coatings include photosensitive components dispersed within an organic polymeric binder. After a portion of the coating is exposed to radiation (commonly referred to as imagewise exposure), the exposed portion becomes either more developable or less developable in a particular liquid than an unexposed portion of the coating. A printing plate is generally considered a positive-working plate if, after exposure to radiation, the exposed portions or areas of the radiation-sensitive coating become more developable and are removed in the developing process to reveal the hydrophilic surface. Conversely, the plate is considered a negative-working plate if the exposed portions or areas become less developable in the developer and the unexposed portions or areas are removed in the developing process. After being developed in a suitable liquid, the coating areas (i.e. image area) that remain on the plate provide an ink-receptive image, while the revealed regions of the substrate's hydrophilic surface repel ink.
Radiation exposure of imaging layers is generally performed using either ultraviolet, infrared (“IR”) or visible radiation. IR radiation exposure (as well as other types of radiation exposure) may be advantageously utilized in an imaging technique referred to herein as “direct-write” imaging. Direct-write imaging using infrared radiation is a process in which a thermally sensitive coating of a printing plate precursor is exposed to infrared radiation from a laser source. More particularly, a computer-controlled infrared laser imagewise exposes small regions of the thermally sensitive composition to produce an image area pixel-by-pixel. Examples of plates prepared by this process are reported in U.S. Pat. No. 5,372,915 (Haley et al.). These plates include an imaging layer comprising a mixture of dissolvable polymers and an infrared radiation absorbing compound. Although the reported plates utilize direct writing techniques, the imaged plates must still be developed in an alkaline solution prior to mounting on a press.
It has further been recognized that such direct writing techniques may be utilized in the formation of “processless” printing plates. As used herein, the term “processless” refers to printing plate precursors that do not require one or more conventional processing steps (e.g. development) prior to mounting on a printing press.
One method for forming processless printing plates is through ablation of a thermally sensitive layer. For example, Canadian 1,050,805 (Eames) reports a dry planographic printing plate comprising an ink receptive substrate, an overlying silicone rubber layer, and an interposed layer containing laser energy absorbing particles (such as carbon particles) in a self-oxidizing binder (such as nitrocellulose). When such plates are exposed to focused, near-IR radiation with a laser, the absorbing layer converts the infrared energy to heat thus partially loosening, vaporizing or ablating the absorber layer and the overlying silicone rubber. Similar plates are reported in Research Disclosure 19201 (1980) as having vacuum-evaporated metal layers to absorb laser radiation in order to facilitate the removal of a silicone rubber overcoat layer. These plates are developed by wetting with hexane and rubbing. Additional patents reporting ablatable printing plates include U.S. Pat. No. 5,385,092 (Lewis et al.), U.S. Pat. No. 5,339,737 (Lewis et al.), U.S. Pat. No. 5,353,705 (Lewis et al.), U.S. Re. Pat. No. 35,512 (Nowak et al.), and U.S. Pat. No. 5,378,580 (Leenders).
Ablatable printing plates have a number of disadvantages. The process of ablation tends to produce debris and vaporized materials in the image setting equipment, which must consequently be collected. Also, the laser intensity or power required for ablation may be very high, and the components of such printing plates may be expensive, difficult to use, possess a reduced life, and may produce an unacceptable printing quality.
Thermal or laser mass transfer is another method of preparing processless lithographic printing plates. Such methods are reported, for example, in U.S. Pat. No. 5,460,918 (Ali et al.) wherein a hydrophobic image is transferred from a donor sheet to a microporous hydrophilic crosslinked silicated surface of the receiver sheet. U.S. Pat. No. 3,964,389 (Peterson) reports a process of laser transfer of an image from a donor material to a receiver material requiring a high temperature post-heating step.
EP-A 0 652 483 (Ellis et al.) reports processless lithographic printing plates that are imageable using IR lasers, and that do not require wet processing prior to mounting on a press. These plates comprise an imaging layer that becomes more hydrophilic upon imagewise exposure to heat. This coating contains a polymer having pendant groups (such as t-alkyl carboxylates) that are capable of reacting under heat or acid to form more polar, hydrophilic groups.
U.S. Pat. Nos. 6,482,571 and 6,548,222 to Teng report on-press developable printing plates having a thermosensitive layer including a free radical initiator, a radiation absorbing material and a polymerizable monomer.
More recently, it has been determined that thermally sensitive coatings containing cyanoacrylate polymers may be particularly useful in the formation of processless printing plates. For example, U.S. Pat. No. 5,605,780 (Burberry et al.) reports printing plates that are imaged by an ablation method whereby exposed areas are removed using the heat generated by a focused high-intensity laser beam. The imaging layer is composed of an IR-absorbing compound in a film-forming cyanoacrylate polymer binder. In order for thermal ablation to be successful in such printing plates, the imaging layer thickness is generally less than 0.1 μm and the weight ratio of IR-absorbing compound to the cyanoacrylate polymer is at least 1:1. Thus, the imaging layers are quite thin and have a significant amount of expensive IR-absorbing compound.
Additionally, U.S. Pat. No. 6,551,757, incorporated herein by reference, reports the use of cyanoacrylate polymers in processless printing plates, in which, after exposure to infrared radiation, imaged regions may be developed “on press” by contacting an imaged thermally sensitive layer containing the cyanoacrylate polymer with aqueous fountain solution.
Although the '780 patent and U.S. patent application Ser. No. 09/864,570 filed May 24, 2001 report the benefits of using cyanoacrylate polymers in thermally sensitive layers of printing plates (e.g. ink affinity, adhesion, wear characteristics), these reported printing plates using cyanoacrylate polymers may tend to suffer from certain drawbacks. First, the reported thermally sensitive layers may provide a discontinuous coating, revealing bare patches of substrate. Second, the reported thermally sensitive layers may produce unsatisfactory levels of ablation during exposure of the plates to IR radiation. Further yet, coatings of this type may suffer from problems with background sensitivity, as well as background scumming.
Thus, it would be desirable to prepare a processless, negative-working lithographic printing plate, which maintains the beneficial characteristics of utilizing cyanoacrylate polymers in thermally sensitive layers, but improves upon or overcomes one or more of the aforementioned drawbacks of using cyanoacrylate polymers.